This invention relates to water-swellable polymer microgels and to methods for their use.
It has been known for some time to employ water-soluble polymers such as polyacrylamide as thickening agents, e.g., as taught in Encyclopedia of Polymer Science and Technology, Interscience Publishers, Vol. 1, 192 (1964), and as agents for restricting the flow of liquids through subterranean formations, e.g., as taught in U.S. Pat. No. 3,039,529.
Normally such polymers which are generally linear are advantageously prepared by a microdisperse polymerization technique such as described in U.S. Pat. Nos. 3,284,393 and 2,982,749. Unfortunately, these linear polymers exhibit virtually no gel strength, i.e., do not resist viscosity changes as a result of mechanical working or milling. As a result, when such polymers are subjected to the high shearing action that is common to many applications wherein such polymers are used as thickening agents or as agents for restricting the flow of liquids through pore structures, they undergo substantial degradation in molecular weight, thereby impairing most of their desirable properties.
Attempts to improve the gel strength of such polymers via cross-linking such polymers as shown in U.S. Pat. No. 3,247,171 have not been very successful. Accordingly, heretofore in order to obtain the desired increase in viscosity or fluid mobility control with the aforementioned polymers, it has been necessary to employ rather low shearing, mixing or pumping apparatus in the desired applications. Such low shearing apparatus are generally less economical and more time consuming to employ.
In view of the aforementioned shear sensitivity of the aforementioned water-soluble, linear polymers and water-swellable, cross-linked polymers, it would be highly desirable to provide a polymer capable of imparting substantial viscosity to an aqueous medium but which will resist degradation when such aqueous medium is subjected to the high shear common to many mixing and pumping apparatus.